1. Field of the Invention
The present invention relates to a metal--insulator--semiconductor (MIS) type semiconductor device using a compound semiconductor and a method for fabricating the same, and more particularly relates to an MIS type semiconductor device using a GaAs semiconductor and a method for fabricating the same.
2. Description of the Related Art
As an MIS type semiconductor device using a compound semiconductor substrate made of GaAs, various types of devices have been proposed. In the proposed MIS type semiconductor devices, silicon oxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4), aluminum oxide (Al.sub.2 O.sub.3) or the like is used as an insulating layer. Moreover, the use of an oxide film which is obtained by oxidizing a GaAs substrate with plasma or a semiconductor layer made of AlGaAs as an insulating layer has been reported.
However, in an MIS type semiconductor device using such a conventional insulating layer, an interface state having a density of 10.sup.12 cm.sup.-2 eV.sup.-1 or more occurs on a surface of the GaAs substrate in contact with the insulating layer. The interface state exists because a large number of surface states, which are generated by donors or acceptors, are present on the surface of the substrate. The donors are generated since As atoms (As.sub.Ga) are positioned at Ga lattice sites on the surface of the GaAs substrate, and the acceptors are generated since Ga atoms (Ga.sub.As) are positioned at As lattice sites on the surface of the GaAs substrate. If electrons or holes are trapped in the thus generated interface state, it becomes difficult to form an inversion layer which plays an important part for operating as an MIS semiconductor device in the GaAs substrate.
On the other hand, since the density of the interface state generated on the surface of the GaAs substrate in a conventional MIS type semiconductor device using an AlGaAs semiconductor is reduced to that on the surface of the silicon semiconductor substrate of the MOS type semiconductor device, it is expected that an MIS semiconductor device having excellent characteristics can be fabricated by using AlGaAs. However, since the maximum value of the energy gap of AlGaAs is about 2 eV, a leak current increases in accordance with the increase in a bias voltage. Therefore, an MIS type semiconductor device using an AlGaAs semiconductor may not be applied with a larger operation voltage than that applied to a conventional MOS type semiconductor device.
Thus, a conventional MIS type semiconductor device using the GaAs Substrate is not suitable for practical use as a product.
The present invention can solve the above problems, and its objective is to provide an MIS type semiconductor device having excellent characteristics as a device by forming a boundary of GaAs/insulating layer having a low interface state density and a method for fabricating the same.